From a physiological perspective, the human body comprises a set of interacting systems, each having specific functions and purposes. These systems maintain the body's internal stability by coordinating the response of its parts to any situation or stimulus that would tend to disturb its normal condition or function. The nervous system includes the central nervous system and the peripheral nervous system. The central nervous system is the brain and the spinal cord. The musculoskeletal system includes the skeleton and attached muscles and includes bones, ligaments, tendons, and cartilage. The circulatory system includes the heart and blood vessels, including arteries, veins and capillaries. The respiratory system includes the nose, trachea and lungs. The gastrointestinal system includes the mouth, esophagus, stomach, intestines, liver, pancreas and gallbladder. The integumentary system includes the skin, hair, nails, sweat glands and sebaceous glands. The urinary system includes the kidneys and bladder. The immune system includes white blood cells, thymus and lymph nodes. The endocrine system includes the pituitary, thyroid, adrenal and parathyroid glands.
Various sensors may be applied for analyzing and measuring the processes occurring in the above-cited physiological systems and for generating physiological parameters indicative of health or wellness as a result. As one example, a pulse oximetry sensor generates a blood-volume plethysmograph waveform from which oxygen saturation of arterial blood and pulse rate may be determined, among other parameters. As another example, an acoustic sensor may be used to detect airflow sounds in the lungs, bronchia or trachea, which are indicative of respiration rate.